Synchronization of a decoding and presentation process for received bitstreams is a particularly important aspect of real-time digital data delivery systems. For example, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1 refers to ISO/IEC standards 11172-1 (Systems), 11172-2 (Video), 11172-3 (Audio), 11172-4 (Compliance Testing), and 11172-6 (Technical Report). The second, known as MPEG-2, refers to ISO/IEC standards 13818-1 (Systems), 13818-2 (Video), 13818-3 (Audio), 13818-4 (Compliance).
The MPEG standards address the timing and synchronization issues for decoders of MPEG data streams (e.g., video, audio, data, and the like) as follows. A sample of a 27 MHz clock is transmitted in a program clock reference (PCR) field of a transport stream packet. The PCR indicates a time when the transport decoder is expected to have completed reading the PCR field. The phase of the local clock running at the decoder is compared to the PCR value in the bit stream at the instant at which it is obtained to determine whether the decoding process is synchronized.
In general, the PCR from the transport bit stream does not directly change the phase of the system clock of the decoder, but only serves as an input to adjust the clock rate of a Voltage Controlled Crystal Oscillator (VCXO). This adjustment is accomplished by comparing the PCR sample to the output frequency of the VCXO and responsively adjusting the VCXO until the two frequencies match. The system clock of the decoder is then derived from the VCXO. Thus, the system clock of the decoder is adjusted and locked to the 27 MHz clock of the encoder or transmitter via the transmitter clock sample included in the PCR.
A disadvantage of the prior art decoder arrangement is the relatively high cost of the VCXO. Therefore, a need exists in the art for a method and apparatus which is able to perform timing functions in a data delivery system using an inexpensive oscillator, e.g., a substantially fixed frequency oscillator.